A roll over detector as described in the opening paragraph is known from JP 2002-200951 A, in which a type of roll over phenomenon of a vehicle is identified based on a roll rate and a lateral acceleration and then a threshold for detecting a roll over phenomenon is changed according to the identified type of roll over phenomenon. With this known roll over detector, since the threshold can be adapted to the respective types of roll over phenomena, it is possible to improve the accuracy of detection of the roll over phenomenon.
By the way, of the actual phenomena that occur when the vehicle rolls, there is a reacting roll phenomenon. The reacting roll phenomenon occurs when the vehicle rolls largely in one direction but doesn't roll over, restores to its horizontal state and then rolls in an opposite direction in reaction to the roll phenomenon in one direction. For this reason, the threshold for detecting a roll over phenomenon must be set so as not to react to this reacting roll phenomenon. In connection with this, in the known roll over detector, since the threshold for detecting a roll over phenomenon is changed without considering the reacting roll phenomenon, there is a possibility that the threshold for detecting a roll over phenomenon might be exceeded when the reacting roll phenomenon occurs. On the other hand, if the threshold is set high enough not to react to the reacting roll phenomenon, it leads to a delay in detection when the roll over phenomenon actually occurs.